Date of Award

2023

Document Type

Open Access Master's Thesis

Degree Name

Master of Science in Applied Ecology (MS)

Administrative Home Department

College of Forest Resources and Environmental Science

Advisor 1

Sarah R. Hoy

Committee Member 1

John Vucetich

Committee Member 2

Carsten Kulheim

Abstract

To adequately manage forested ecosystems in the face of climate change, there is an urgent need to study how tree growth and biochemistry vary in response to changes in weather and levels of herbivory across different landscape types. However, few studies have assessed how both free-ranging herbivore populations and weather influence the foliar chemistry of boreal tree species over relatively large temporal and spatial scales. In this study, I assessed the extent to which the nitrogen (N) content and the carbon-to- nitrogen (C/N) ratio in the foliage of Balsam fir (Abies balsamea), a key boreal forest species, was influenced by variation in moose (Alces alces) browsing pressure, weather and landscape characteristics over a 17-year period. Specifically, I assessed how the N content (%) and C/N ratio in balsam fir foliage varied in response to: (1) the severity of moose browsing pressure on individual saplings over their lifetime, (2) interannual variation in moose density, (3) landscape characteristics including soil type, bedrock type, and habitat type, and (4) interannual variation in several weather variables including seasonal and annual variation in temperature, precipitation, and the North Atlantic Oscillation (NAO) index. The N and C/N ratio in balsam fir foliage were not closely associated with any of the weather variables examined. Furthermore, bedrock type and soil type explained only a relatively small portion (<10%) of the spatial variance in N and C/N. By contrast, the N content and C/N ratio were strongly related to browse pressure and moose density. Moose density explained approximately half of the interannual variation in N and C/N ratio, with saplings tending to have a higher N and a lower C/N ratio following years of high moose densities. This result may reflect that saplings increased the production of nitrogen-based Plant Secondary Metabolites (PSM) as a defense mechanism to deter future browsing. This finding could also reflect an increase in photosynthetic rates and growth in the year following high moose densities or after being severely browsed. Together these results suggest that the influence of moose herbivory on the foliar chemistry of balsam fir is significant and should be considered in future forest management plans.

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